The present invention relates to structures and methods for providing electrical contacts to surfaces of semiconductor materials, and in particular, to thermally stable metallization systems for contacting photovoltaic devices. The continuing development of semiconductor devices, and of photovoltaic devices in particular, has resulted in various methods for optimizing the efficiency and lifetime of devices operating at elevated temperatures. For example, photovoltaic devices are often used in space applications where thermal and radiation damage due to the environment can severely reduce device lifetime and efficiency.
The interdiffusion of the material used as an electrical conductor with the photoactive surface of photovoltaic devices from which current is collected has resulted in the development of diffusion barrier schemes to control the instability of adjacent layers. This interdiffusion, resulting in the breakdown of the structure and loss of efficiency, is particularly deleterious in devices exposed to high temperatures. A large number of electronic and optoelectronic devices, including photovoltaic cells, are designed to operate at temperatures in the range of -100.degree. C. to 100.degree. C. However, hostile environmental factors require the devices to endure occasional temperature elevation up to or exceeding 700.degree. C. Even if these temperature excursions are limited to periods of several minutes, the aforementioned interdiffusion of the semiconductor and metal system is highly deleterious.
Therefore, there exists a need for the development of thermally stable metallization systems employing barriers to prevent the interdiffusion of conductive metal and the semiconductor materials from which current is collected.
Many nitrides, borides, and carbides of transition metals have been suggested for use as diffusion barriers with silicon. The use of barriers to prevent the diffusion of an overlying conductive layer into an ohmic contact metallized layer on GaAs wafers has been explored in an effort to improve the survivability of such metallized photovoltaic devices at temperatures up to 700.degree. C.
The grid pattern used in photovoltaic devices contains several loss mechanisms which reduce the available power output. It is desirable to reduce the grid area as the grid blocks out light that would otherwise enter the cell. This factor must be balanced against ohmic losses that are reduced with greater area coverage by the grid contact.
Even with the many known device improvements, photovoltaic structures continue to suffer efficiency losses due to mechanical and thermal stresses encountered at elevated temperatures. Cells using compound semiconductors in particular, lose their structural and chemical integrity due to decomposition, especially at the higher operating temperatures.